3C 286: a bright, compact, stable, and highly polarized calibrator for millimeter-wavelength observations

Context. Several millimeter and submillimeter facilities with linear polarization observing capabilities have started operating during the last years. These facilities, as well as other previous millimeter telescopes and interferometers, require bright and stable linear polarization calibrators to calibrate new instruments and to monitor their instrumental polarization. The current limited number of adequate calibrators implies difficulties in the acquisition of these calibration observations. Aims. Looking for additional linear polarization calibrators in the millimeter spectral range, we started monitoring 3C 286 in mid-2006. This source is a standard and highly stable polarization calibrator for radio observations. Methods. Here we present the 3mm and 1mm monitoring observations obtained between September 2006 and January 2012 with the XPOL polarimeter on the IRAM 30m Millimeter Telescope. Results. Our observations show that 3C 286 is a bright source of constant total flux with 3mm flux density S-3mm = (0.91 +/- 0.02) Jy. The 3mm linear polarization degree (p(3mm) = [13.5 +/- 0.3]%) and polarization angle (chi(3mm) = [37.3 +/- 0.8]degrees, expressed in the equatorial coordinate system) were also constant during the time span of our observations. Although with poorer time sampling and signal-to-noise ratio, our 1mm observations of 3C 286 are also reproduced by a constant source of 1mm flux density (S-1mm = [0.30 +/- 0.03] Jy), polarization fraction (p(1mm) = [14.4 +/- 1.8]%), and polarization angle (chi(1mm) = [33.1 +/- 5.7]degrees). Conclusions. This, together with the previously known compact structure of 3C 286 - extended by similar to 3.5 '' in the sky - allow us to propose 3C 286 as a new calibrator for both single-dish and interferometric polarization observations at 3mm, and possibly at shorter wavelengths.